Testosterone (T) is a primary androgenic hormone produced in the interstitial cells of the testes and is responsible for normal growth, development and maintenance of male sex organs and secondary sex characteristics (e.g., deepening voice, muscular development, facial hair, etc.). Throughout adult life, testosterone is necessary for proper functioning of the testes and its accessory structures, prostate and seminal vesicle; for sense of well-being; and for maintenance of libido, erectile potency.
Testosterone deficiency—insufficient secretion of T characterized by low serum T concentrations—can give rise to medical conditions (e.g., hypogonadism) in males. Symptoms associated with male hypogonadism include impotence and decreased sexual desire, fatigue and loss of energy, mood depression, regression of secondary sexual characteristics, decreased muscle mass, and increased fat mass. Furthermore, hypogonadism in men is a risk factor for osteoporosis, metabolic syndrome, type II diabetes and cardiovascular disease.
Various testosterone replacement therapies are commercially available for the treatment of male hypogonadism. Pharmaceutical preparations include both testosterone and testosterone derivatives in the form of intramuscular injections, implants, oral tablets of alkylated T (e.g., methyltestosterone), topical gels, or topical patches. All of the current T therapies, however, fail to adequately provide an easy and clinically effective method of delivering T. For example, intramuscular injections are painful and are associated with significant fluctuations in serum T levels between doses; T patches are generally associated with levels of T in the lower range of normal (i.e., clinically ineffective) and often cause substantial skin irritation; and T gels have been associated with unsafe transfer of T from the user to women and children. As well, the sole “approved” oral T therapy, methyltestosterone, is associated with a significant occurrence of liver toxicity. Over time, therefore, the current methods of treating testosterone deficiency suffer from poor compliance and thus unsatisfactory treatment of men with low T.
Testosterone and its esters are poorly bioavailable—owing to extensive first pass intestinal and hepatic metabolism—or ineffective—due to an inability of the body to liberate testosterone from its testosterone prodrug. For example, testosterone and testosterone esters with side chains of less than 10 carbons in length are primarily absorbed via the portal circulation resulting in substantial, if not total, first pass metabolism. Fatty acid esters of long carbon chains (i.e., 14 or more carbons) may be absorbed by intestinal lymphatics, but the longer the fatty acid chain length, the slower the rate and extent of hydrolysis of the ester by esterases to liberate testosterone thus resulting in poor (i.e., clinically ineffective) pharmacological activity.
Other than selection of a testosterone ester, the formulation of the testosterone ester presents unique challenges. The gastrointestinal environment is decidedly aqueous in nature, which requires that drugs must be solubilzed for absorption. However, testosterone and particularly its esters are extremely insoluble in water and aqueous media, and even if the T or T ester is solubilized initially in the formulation, the formulation must be able to maintain the drug in a soluble or dispersed form without precipitation or, otherwise, coming out of solution in vivo (although such a property can be tested in vitro, for example, by mixing the contents of a formulation in simulated intestinal fluid). Furthermore, an oral T formulation must, effectively release T or T ester according to a desired release profile. Hence, an effective formulation of T or T ester must balance good solubility with optimum release and satisfaction of a targeted plasma or serum concentration profile.
For these reasons, among others, no oral formulation of testosterone or testosterone esters has been approved by the United States Food and Drug Administration (FDA) to date. In fact, the only oral testosterone product ever approved to date by the FDA is methyltestosterone (in which a methyl group covalently bound to a testosterone “nucleus” at the C-17 position to inhibit hepatic metabolism; note, also, that methyltestosterone is not a prodrug of testosterone) and this approval occurred several decades ago. Unfortunately, use of methyltestosterone has been associated with a significant incidence of liver toxicity, and it is rarely prescribed to treat men with low testosterone.
As noted above, fatty acid esters of testosterone provide yet another mode of potential delivery of testosterone to the body (i.e., as a “prodrug”). Once absorbed, testosterone can be liberated from its ester via the action of non-specific tissue and plasma esterases. Furthermore, by increasing the relative hydrophobicity of the testosterone moiety and the lipophilicity of the resulting molecule as determined by its n-octanol-water partition coefficient (log P) value, such prodrugs can be absorbed, at least partially, via the intestinal lymphatics, thus reducing first-pass metabolism by the liver. In general, lipophilic compounds having a log P value of at least 5 and oil solubility of at least 50 mg/mL are transported primarily via the lymphatic system.
Despite their promise, prodrugs of testosterone, including testosterone esters, have not been formulated in a manner to achieve effective and sustained serum testosterone levels at eugonadal levels (i.e., average serum T concentration falling in the range of about 300-1100 ng/dL). In fact, an orally administered pharmaceutical preparation of a testosterone prodrug, including testosterone esters, has yet to be approved by the FDA.
Hence, there remains a need for an oral formulation of a testosterone ester, which provides optimum serum testosterone levels that are clinically effective to treat hypogonadal men (i.e., those with a serum T concentration of ≤300 ng/dL) over an extended period of time.